54 research outputs found
The age of the oldest Open Clusters
We determine ages of 71 old Open Clusters by a two-step method: we use
main-squence fitting to 10 selected clusters, in order to obtain their
distances, and derive their ages from comparison with our own isochrones used
before for Globular Clusters. We then calibrate the morphological age indicator
delta(V), which can be obtained for all remaining clusters, in terms of age and
metallicity. Particular care is taken to ensure consistency in the whole
procedure. The resulting Open Cluster ages connect well to our previous
Globular Cluster results. From the Open Cluster sample, as well as from the
combined sample, questions regarding the formation process of Galactic
components are addressed. The age of the oldest open clusters (NGC6791 and
Be17) is of the order of 10 Gyr. We determine a delay by 2.0+-1.5 Gyr between
the start of the halo and thin disk formation, whereas thin and thick disk
started to form approximately at the same time. We do not find any significant
age-metallicity relationship for the open cluster sample. The cumulative age
distribution of the whole open cluster sample shows a moderately significant
(~2sigma level) departure from the predictions for an exponentially declining
dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution
does not show any trend with galactocentric distance, but the clusters with
larger height to the Galactic plane have an excess of objects between 2-4 and 6
Gyr with respect to their counterpart closer to the plane of the Galaxy.Comment: Astronomy & Astrophysics, in pres
The impact of systematic uncertainties in stellar parameters on integrated spectra of stellar populations
In this paper we investigate a hitherto unexplored source of potentially
significant error in stellar population synthesis (SPS) models, caused by
systematic uncertainties associated with the three fundamental stellar
atmospheric parameters; effective temperature T_eff, surface gravity g, and
iron abundance [Fe/H]. All SPS models rely on calibrations of T_eff, logg and
[Fe/H] scales, which are implicit in stellar models, isochrones and synthetic
spectra, and are explicitly adopted for empirical spectral libraries. We assess
the effect of a mismatch in scales between isochrones and spectral libraries
(the two key components of SPS models) and quantify the effects on 23 commonly
used diagnostic line indices. We find that typical systematic offsets of 100K
in T_eff, 0.15 dex in [Fe/H] and/or 0.25 dex in logg significantly alter
inferred absolute ages of simple stellar populations (SSPs) and that in some
circumstances, relative ages also change. Offsets in T_eff, logg and [Fe/H]
scales for a scaled-solar SSP produce deviations from the model which can mimic
the effects of altering abundance ratios to non-scaled-solar chemical
compositions, and could also be spuriously interpreted as evidence for a more
complex population, especially when multiple-index or full-SED fitting methods
are used. We stress that the behavior we find can potentially affect any SPS
models, whether using full integrated spectra or fitting functions to determine
line strengths. We present measured offsets in 23 diagnostic line indices and
urge caution in the over-interpretation of line-index data for stellar
populations.Comment: 14 pages, including 4 figures and 3 tables. Accepted for publication
in Ap
An empirical test of the theoretical population corrections to the Red Clump absolute magnitude
The mean absolute magnitude of the local red clump (RC) is a very well
determined quantity due to the availability of accurate HIPPARCOS parallaxes
for several hundred RC stars, potentially allowing it to be used as an accurate
extra-galactic distance indicator. Theoretical models predict that the RC mean
magnitude has non-linear dependencies on both age and metallicity. This
suggests that a population correction, based on the star formation rate (SFR)
and age-metallicity relation (AMR) of a particular system, should be applied to
the local RC magnitude before it can be compared to the RC in that system in
order to make a meaningful distance determination. Using a sample of 8 Galactic
open clusters and the GC 47 Tuc, we determine the cluster distances, and hence
the RC absolute magnitude in V, I and K, by applying our empirical main
sequence fitting method, which utilizes a large sample of local field dwarfs
with accurate HIPPARCOS parallaxes. The age and metallicity range of these 9
clusters enable us to make a quantitative assessment of the age and metallicity
dependencies of the population corrections predicted by the theoretical models
of Girardi & Salaris (2001). We find excellent agreement between the empirical
data and the models in all 3 pass-bands, with no statistically significant
trends or offsets, thus fully confirming the applicability of the models to
single-age, single-metallicity stellar populations. Since, from the models, the
population correction is a complicated function of both metallicity and age, if
this method is used to derive distances to composite populations, it is
essential to have an accurate assessment of the SFR and AMR of the system in
question, if errors of several tenths of a magnitude are to be avoided.Comment: 9 pages, 5 figures. MNRAS accepte
A large stellar evolution database for population synthesis studies. IV. Integrated properties and spectra
This paper is the 4th in a series describing the latest additions to the
BaSTI stellar evolution database, which consists of a large set of homogeneous
models and tools for population synthesis studies. Here we present a new set of
low and high resolution synthetic spectra based on the BaSTI stellar models,
covering a large range of simple stellar populations (SSPs) for both scaled
solar and alpha-enhanced metal mixtures. This enables a completely consistent
study of the photometric and spectroscopic properties of both resolved and
unresolved stellar populations, and allows us to make detailed tests on their
integrated properties. Our low resolution spectra are suitable for deriving
broadband magnitudes and colors in any photometric system. These spectra cover
the full wavelength range (9-160000nm) and include all evolutionary stages up
to the end of AGB evolution. Our high resolution spectra are suitable for
studying the behaviour of line indices and we have tested them against a large
sample of Galactic globular clusters. We find that the range of ages, iron
abundances [Fe/H], and degree of alpha-enhancement predicted by the models
matches observed values very well. We have also tested the global consistency
of the BaSTI models by making detailed comparisons between ages and
metallicities derived from isochrone fitting to observed CMDs, and from line
index strengths, for the Galactic globular cluster 47Tuc and the open cluster
M67. For 47Tuc we find reasonable agreement between the 2 methods, within the
estimated errors. From the comparison with M67 we find non-negligible effects
on derived line indices caused by statistical fluctuations, which are a result
of the specific method used to populate an isochrone and assign appropriate
spectra to individual stars. (abridged)Comment: 21 pages including 13 figures. Accepted for publication in ApJ. Low
and high resolution integrated spectra, magnitudes, and mass-to-light ratios
will appear on the BaSTI website by 1st November 2008 - see
http://193.204.1.62/index.htm
Modelling realistic horizontal branch morphologies and their impact on spectroscopic ages of unresolved stellar systems
The presence of an extended blue horizontal branch (HB) in a stellar
population is known to affect the age inferred from spectral fitting to stellar
population synthesis models. However, most population synthesis models still
rely on theoretical isochrones which do not include realistic modelling of
extended HBs. In this work, we create detailed models for a range of old simple
stellar populations (SSPs), to create a variety of realistic HB morphologies,
from extended red clumps, to extreme blue HBs. We achieve this by utilising
stellar tracks from the BaSTI database and implementing a different mass loss
prescription for each SSP created, resulting in different HB morphologies. We
find that, for each metallicity, there is some HB morphology which maximises
Hbeta, making an underlying 14Gyr population look ~5-6Gyr old for the low and
intermediate metallicity cases, and as young as 2Gyr for a solar metallicity
SSP. We explore whether there are any spectral indices capable of breaking the
degeneracy between an old SSP with extended blue HB and a truly young or
intermediate age SSP, and find that the CaII index of Rose(1984) and the
strength of the MgII doublet at 2800A are promising candidates, in combination
with Hbeta and other metallicity indicators such as Mgb and Fe5406. We also run
Monte Carlo simulations to investigate the level of statistical fluctuations in
the spectra of typical stellar clusters. We find that fluctuations in spectral
indices are significant even for average to large globular clusters, and that
various spectral indices are affected in different ways, which has implications
for full-spectrum fitting methods. Hence we urge caution if these types of
stellar clusters are to be used as empirical calibrating objects for various
aspects of SPS models. (Abridged)Comment: Accepted for publication in MNRAS. 10 pages, including 7 figure
The ion seeps tonight: Assessing ionic transport in multilayered nanocomposites
Figure 6 – Schematic of cation (M+) transport through an organized multilayered composite.
Controlling ion transport across membranes and interfaces is one of the central themes challenging technological pursuits ranging from corrosion to energy storage and chemical separations. Here, we present several examples in which we have studied the application of multilayer nanocomposites to regulate ion transport. These composites comprise organized layers of functional or structural elements, integrated within composites such that the specific nanostructure and composition of the materials play important roles in defining ionic interactions and mobility. In cases such as corrosion inhibition, thin film composite coatings are intended to block ionic transport, retarding deleterious corrosion reactions. We show that by manipulating the materials chemistry of highly organized polymer clay nanocomposite thin film barriers, it is possible to significantly increase corrosion resistance of steel samples in a simulated sea water environment. In contrast, for energy storage applications such as batteries, composite separators capable of rapid ionic diffusion are desired for high current performance. We explore how layered composite structures may provide effective ion diffusion planes, leading to promising ionic conductivity in new solid state separators. Finally, in chemical separations, the selective transport of ions becomes important. We examine how manipulating the chemical and electrostatic composition of layered polyelectrolyte materials leads to preferential cation transport through these composite structures, a key property for an effective separations membrane. These different technologies exemplify how the principles governing ion transport through multilayered materials can be adapted for widely varied applications, and they illustrate the potential for this materials development strategy to enable new classes of functional composite materials.
Please click Additional Files below to see the full abstract
Resolving the 47 Tucanae Distance Problem
We present new B, V and I-band photometry for a sample of 43 local subdwarfs
with HIPPARCOS parallax errors < 13%, in the metallicity range -1.0 < [Fe/H] <
-0.3, which we use to perform main sequence (MS) fitting to the Galactic
globular cluster 47 Tuc. This sample is many times larger than those used in
previous MS-fitting studies and also enables us to fit in two colour planes,
V/(B-V) and V/(V-I). With this enlarged subdwarf sample we investigate whether
the current discrepancy in empirical distance estimates for 47 Tuc, arising
from recent MS-fitting and white dwarf fitting results, is due to inaccuracies
in the MS-fitting method. Comparison of published photometries for 47 Tuc has
revealed systematic offsets which mean that the main-line used in
previous studies may be too blue by ~ 0.02 mag. We have derived main-lines in
V/(B-V) and V/(V-I) from the data of Kaluzny et al. (1998), which we have
recalibrated from the `secondary' standards in 47 Tuc of Stetson (2000). Using
an assumed cluster reddening of E(B-V)=0.04, our best-fit apparent distance
modulus is (m-M)V=13.37 (+0.10)(-0.11) in both colour planes, which implies a
cluster age of 11.0 +/-1.4 Gyr and leads to a dereddened distance modulus of
(m-M)0=13.25 (+0.06)(-0.07). Comparison with previous work shows that our
apparent distance modulus is ~0.2 mag smaller than those derived in previous
MS-fitting studies. The difference is accounted for by our preferred cluster
reddening and the recalibration of the cluster photometry, which has made the
main-line redder by an average of 0.02 mag in (B-V). Independent support for
our MS-fitting distance comes from consideration of the Red Clump in the
cluster, from which we derive a dereddened distance modulus of (m-M)0=13.31
+/-0.05, which is in agreement with the MS-fitting result. (Abridged)Comment: 23 pages, including 2 tables and 4 figures. ApJ accepte
Galaxy and Mass Assembly (GAMA): Variation in galaxy structure across the green valley
Using a sample of 472 local Universe (z \u3c 0.06) galaxies in the stellar mass range 10.25 \u3c logM*/M⊙ \u3c 10.75, we explore the variation in galaxy structure as a function of morphology and galaxy colour. Our sample of galaxies is subdivided into red, green, and blue colour groups and into elliptical and non-elliptical (disk-type) morphologies. Using Kilo- Degree Survey (KiDS) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Kilo-Degree Infrared Galaxy Survey (VIKING) derived postage stamp images, a group of eight volunteers visually classified bars, rings, morphological lenses, tidal streams, shells, and signs of merger activity for all systems. We find a significant surplus of rings (2.3s) and lenses (2.9s) in disk-type galaxies as they transition across the green valley. Combined, this implies a joint ring/lens green valley surplus significance of 3.3s relative to equivalent disk-types within either the blue cloud or the red sequence. We recover a bar fraction of ~44 per cent which remains flat with colour, however, we find that the presence of a bar acts to modulate the incidence of rings and (to a lesser extent) lenses, with rings in barred disk-type galaxies more common by ~20-30 percentage points relative to their unbarred counterparts, regardless of colour. Additionally, green valley disk-type galaxies with a bar exhibit a significant 3.0s surplus of lenses relative to their blue/red analogues. The existence of such structures rules out violent transformative events as the primary end-of-life evolutionary mechanism, with a more passive scenario the favoured candidate for the majority of galaxies rapidly transitioning across the green valley
The Host Galaxies of AGN
We examine the properties of the host galaxies of 22,623 narrow-line AGN with
0.02<z<0.3 selected from a complete sample of 122,808 galaxies from the Sloan
Digital Sky Survey. We focus on the luminosity of the [OIII]5007
emission line as a tracer of the strength of activity in the nucleus. We study
how AGN host properties compare to those of normal galaxies and how they depend
on L[OIII]. We find that AGN of all luminosities reside almost exclusively in
massive galaxies and have distributions of sizes, stellar surface mass
densities and concentrations that are similar to those of ordinary early-type
galaxies in our sample. The host galaxies of low-luminosity AGN have stellar
populations similar to normal early-types. The hosts of high- luminosity AGN
have much younger mean stellar ages. The young stars are not preferentially
located near the nucleus of the galaxy, but are spread out over scales of at
least several kiloparsecs. A significant fraction of high- luminosity AGN have
strong H absorption-line equivalent widths, indicating that they
experienced a burst of star formation in the recent past. We have also examined
the stellar populations of the host galaxies of a sample of broad-line AGN. We
conclude that there is no significant difference in stellar content between
type 2 Seyfert hosts and QSOs with the same [OIII] luminosity and redshift.
This establishes that a young stellar population is a general property of AGN
with high [OIII] luminosities.Comment: submitted to MNRAS, 44 pages, version with full resolution figures
available at http://www.mpa-garching.mpg.de/~gamk/agnpaper/agnpaper.p
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